It’s good to be a dirty old man. If production of children is the judging criterion, then an ideal match in a monogamous relationship includes a man mated to a woman 4 to 6 years younger than himself. In some cultures, an age difference of 15 years seems optimal. The reasons for this situation are not clear. But, it could be as simple as the fact that young women can have more babies than old women.
NewScientist.com 19 March 2008.
Why Men Should Pair Off With Younger Women.